1. Technology Field
The present invention generally relates to x-ray generating devices. In particular, the present invention relates to an electron shield, configured to intercept and absorb backscattered electrons, having a construction that prevents heat-related damage thereto.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, and then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure, a cathode, and an anode. The cathode, having a filament for emitting electrons, is disposed within the vacuum enclosure, as is the anode that is oriented to receive the electrons emitted by the cathode.
The vacuum enclosure may be composed of metal such as copper, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. The entire outer housing is typically covered with a shielding layer (composed of, for example, lead or similar x-ray attenuating material) for preventing the escape of x-rays produced within the vacuum enclosure. In addition a cooling medium, such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. Depending on the configuration, heat can be removed from the coolant by circulating it to an external heat exchanger via a pump and fluid conduits.
In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by thermionic emission. An electric potential is established between the cathode and anode, which causes the electron stream to gain kinetic energy and accelerate toward a target surface disposed on the anode. Upon impingement at the target surface, some of the resulting kinetic energy in converted to electromagnetic radiation of very high frequency, i.e., x-rays.
The characteristics of the x-rays produced depends in part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”), such as tungsten or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. The resulting x-rays can be collimated so that they exit the x-ray device through predetermined regions of the vacuum enclosure and outer housing for entry into the x-ray subject, such as a medical patient.
One challenge encountered with the operation of x-ray tubes relates to backscattered electrons, i.e., electrons that rebound from the target surface along unintended paths in the vacuum enclosure. These rebounding, backscattered electrons can impact areas of the x-ray tube where such electron impact is not desired. These impacts can either cause excess and possibly damaging heating in the impacted component or result in the creation of “off-focus” x-rays that cloud the x-ray image obtained by the x-ray tube. Either result is undesired.
To minimize the effects of backscattered electrons, an electron shield is often included in x-ray tubes. Interposed between the electron emitting filament of the cathode and the anode target surface, the electron shield includes an aperture through which primary electrons can pass toward impingement on the target surface but is configured to intercept most of the electrons that subsequently backscatter after impingement. The electron shield absorbs a large number of backscattered electrons, thereby preventing their impingement on less desirable portions of the x-ray tube.
Due to the characteristics of tube design, most backscattered electrons intercepted by the electron shield impact the shield about a narrowed portion of the aperture closest the target surface, commonly referred to as the “throat” of the aperture. This results in a relatively large amount of localized electron shield heating about the aperture throat. Known electron shield designs often prove inadequate in handling such heat without causing damage to the electron shield. Indeed, at relatively high x-ray tube power settings, electron shield cracking or other failure at or near the aperture throat can be an all-too common occurrence.
Failure of the electron shield in the manner described above is detrimental to tube performance. In particular, the electron shield often defines a portion of the vacuum envelope in which critical tube components, such as the cathode and anode, are housed. Upon failure of the electron shield, the vacuum can be compromised and x-ray production negatively affected. The x-ray tube can be rendered useless, and must be replaced, often at significant cost.
In light of the above discussion, a need exists for an electron shield that avoids the challenges just described and that acceptably performs at the relatively high power settings common among today's x-ray tube devices.